There's a reason for doing it. That reason is obvious as a brick to the face when you think it through: it gives you a reliable fallback for when your fictive postulates don't cover a situation. That's why this is "exception-based"; you use the real world to determine results when fictional elements cannot or do not handle it.
Let's consider an example, taken from one of my own gaming campaign settings:
- This is a D&D setting, using the classic Basic D&D (Mentzer) ruleset. That right there imposes some unstated postulates.
- Non-humans exist, but are not available as playable characters.
- Gunpowder works, but the technology is not well-developed yet. Firearms are smoothbore flicklock muskets and cannon using black powder.
- Magic exists, but those who use it don't truly comprehend what this power is or how it truly works. (This knowledge is, effectively, a form of treasure.)
- Player-characters come from a not-Europe that just concluded a not-30 Years War, and your character was on the losing side. This is your way out, as a colonist to a land heretofore unknown.
Look, it's this simple. When I don't know what to do, I look at the real historical parallel and apply that as best I can to the matter. That's why I stick with exception-based world-building design. It's the same for my fiction, no matter how fantastic it may be. It's classic--Homer, Tolkien, Howard, and more went this way over the ages--and it works. That's why it's a Best Practice.